<jats:title>SUMMARY</jats:title><jats:p>1. Thromboxane A<jats:sub>2</jats:sub> and prostacyclin are two compounds which have been implicated as important modulators of local cerebral blood flow.<jats:p>2. Concentrations of the stable metabolites of these two compounds, thromboxane B<jats:sub>2</jats:sub> and 6‐keto‐PGF<jats:sub>1</jats:sub>α, were measured in cerebrospinal fluid (CSF) from eight acute ischaemic stroke patients and 14 patients with no evidence of cerebrovascular disease.<jats:p>3. Concentrations of thromboxane B<jats:sub>2</jats:sub> ranged from 0.15 to 4.0 pg/ml and were significantly higher (<jats:italic>P</jats:italic>= 0.025) in the ischaemic stroke group when compared with the control group (0.1–0.3 pg/ml). Simultaneously acquired concentrations of 6‐keto‐PGF<jats:sub>1</jats:sub>α were not elevated in the stroke group when compared to normals.<jats:p>4. These clinical findings support evidence from animal studies that emphasizes the importance of cerebral prostaglandins in mediating the secondary vascular changes of cerebral infarction. In conclusion there is an aberration in CSF thromboxane B<jats:sub>2</jats:sub> concentrations in patients who have had a stroke. This may be an acute or chronic phenomenon.